ESTRO 2021 Abstract Book

S932

ESTRO 2021

Conclusion This observational study showed that mean dose to axilla was highest for level I axilla in both BCS and mastectomy cases. Average (mean) dose to axilla, V20, V40, V45 and V47.5 was higher in post-mastectomy group. Axillary dose was also affected by anatomic and technical parameters such as PTV, BMI, thoracic and body depth, and width. PO-1119 Internal Mammary Irradiation in breast cancer. Do CTV-PTV safety margins matter? C. Hofmann 1 , K.J. Borm 1 , M. Düsberg 1 , M. Oechsner 1 , S.E. Combs 1,2,3 1 Technical University Munich, Medical School, Klinikum rechts der Isar, Department of Radiation Oncology, Munich, Germany; 2 Deutsches Konsortium für Translationale Krebsforschung (DKTK), Partner Site Munich, Munich, Germany; 3 Helmholtzzentrum München, Institute of Radiation Medicine, Munich, Germany Purpose or Objective A recent survey revealed that many radiotherapists are critical towards irradiation of the internal mammary region (IMNI) due to concerns of additional dose to the OARs. Approximately 40% of the radiotherapists therefore "subtract" lung tissue from the PTV when irradiating the lymphatic vessels. Our aim was to evaluate the impact of this subtraction on target coverage and OAR-sparing using a dose accumulation workflow. Materials and Methods From the study collective we selected n=20 patients (11 post-lumpectomy, 9 post-mastectomy) who had received breast/chest wall irradiation including IMNI and regional lymph node irradiation (RNI) with 50.4 Gy á 1.8 Gy treated under daily CBCT imaging. Retrospectively two different target volumes were defined for each patient: a “standard” PTV (PTV_STD) with a 5 mm CTV-PTV safety margin around the lymph node areas and a PTV excluding the overlapping lung volumes from the standard PTV (PTV_ExLung). For both PTVs a treatment plan (VMAT-technique, 6 MV-X) was created and the dosage to the organs at risk (OARs) evaluated. Subsequently the plans were recalculated on the individual CBCTs and the dose distributions were accumulated on the geometry of the corresponding planning CT. Non-rigid registration was performed in Varian Eclipse 15.5, the accumulation in MATLAB 2019b utilizing the image registration framework plastimatch 1.8. Based on the accumulated dose values (DVHs and mean values), the dose coverage in the IMNI-CTV between PTV_STD and PTV_ExLung was compared. Results A comparison of the standard and the amended treatment plan showed that the exclusion of lung tissue from the PTV during IMNI resulted in significant lower planned doses in the heart, LAD, ipsilateral lung and contralateral breast. The cumulative total dose based on the dose distribution in the individual CBCTs showed poorer dose coverage of the IMN-CTV for the PTV_ExLung compared to the PTV_STD: While for the STD treatment plans, the accumulated dose still led to an acceptable dose coverage in the IMN-CTV (mean: 51.4 ± 1.5 Gy; V95%: 95.6 ± 7.4 %; V90%: 99.1 ± 2.7 %), exclusion of lung tissue from the PTV (ExLung) resulted in significant lower values and underdosage in the IMN-CTV (mean: 49.7 ± 1.9 Gy; V95%: 76.6 ± 22.9 %; V90%: 89.6 ± 13.2 %). Conclusion The data shows that reducing the PTV margins by excluding the lung from the PTV, even when performing daily CBCT-based image guidance and position corrections, leads to drastic reduction of dose coverage in the IMN-CTV. A subtraction of the safety margins should therefore not be performed, especially in the case of risk factors (e.g. positive mammaria interna lymph nodes) or when CBCT-based image guidance is performed less frequently. PO-1120 skin toxicity of hypofractionated vs conventional irradiation after mastectomy for breast cancer M. bohli 1,2 , M. el bessi 1 , D. aissaoui 1 , R. ben amor 1 , N. nsiri 1 , J. yahyaoui 1 , A. hamdoun 1 , L. kochbati 1,2 1 abderrahman mami hospital, radiation oncology, ariana, Tunisia; 2 Tunis El Manar University, faculty of medicine, tunis, Tunisia

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